Embryo fossil

Last updated July 13, 2023

Embryo fossils are the preserved remains of unhatched or unborn organisms. Many fossils of the 580 million year old Doushantuo Formation have been interpreted as embryos; embryos are also common throughout the Cambrian fossil record.

Preservation

Taphonomic studies indicate that embryos are preserved for longest in reducing, anoxic conditions. These conditions can keep the embryos intact for long enough for bacteria to mineralise the cells and permit their preservation.^[1] However, phosphatisation was very rapid in the Doushantuo, so it is possible that faster preservation allowed embryos in different regimes to be preserved.

Preservation is mediated by bacterial biofilms. The cells of the embryo break down within hours of their death, under the effect of its own enzymes. Bacteria invade the decaying embryo before its cells can collapse, and biofilms take on and preserve the three-dimensional structure of the cells. The activity of these bacteria promote mineralisation.^[2]

Currently, only fossils representing very early stages in embryonic development have been preserved – no embryos have been interpreted as having undergone a large number cleavage stages.^[3]

Affinities

Some embryos have been interpreted as colonies of sulfur-reducing bacteria, a claim that cannot be upheld in all cases.^[3]

Embryo fossils found in Doushantuo Formation of southern China exhibit occasional asynchronous cell division, common in modern embryos, implying that sophisticated mechanisms for differential cell division timing and embryonic cell lineage differentiation evolved before 551 million years ago. However, embryos composed of hundreds to more than ~1000 cells still show no evidence of blastocoel formation or the organization of blastomeres into epithelia – epithelialization should be underway in modern embryos with >100 cells. Features preserved on Doushantuo embryos are compatible with metazoans (animals), but the absence of epithelialization is consistent only with a stem-metazoan affinity.^[4] It is not until the Cambrian that embryos with demonstrable eumetazoan features occur; gastrulation and a pentaradial symmetry are expressed in some lower Cambrian fossils.^[5]

Some fossil embryos are considered to belong to cnidarians and ecdysozoans, if they even fall into the metazoan crown group. No deuterostome or lophotrochozoan embryos have yet been reported, despite their similar preservation potential; this may be a result of different egg laying behaviour, as ecdysozoans lay eggs in the sediment rather than releasing them into the open – enhancing the chance of them becoming mineralised.^[1]

An alternative interpretation of the embryos appears to discount an animal – or indeed embryonic – interpretation altogether. Cell division without enlargement appears to continue beyond what it would in embryos, and without other embryonic traits becoming apparent. Such division is found in a wide range of eukaryotes, including some that are not truly multicellular, and this more conservative interpretation looks to be more parsimonious than embryonic claims.^[6]

Some have also been interpreted as algal.^[7]

Dinosaurs

Some hadrosaur fossils recovered from the Dinosaur Park Formation may represent embryos.^[8] Titanosaurian and tyrannosaurid embryos are also known.

Related Research Articles

A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood and DNA remnants. The totality of fossils is known as the fossil record.

Bilateria is a group of animals, called bilaterians, with bilateral symmetry as an embryo. This also means they have a head and a tail, as well as a belly and a back. Nearly all are bilaterally symmetrical as adults as well; the most notable exception is the echinoderms, which achieve secondary pentaradial symmetry as adults, but are bilaterally symmetrical during embryonic development.

<span class="mw-page-title-main">Acritarch</span> Microfossils

Acritarchs are organic microfossils, known from approximately 1800 million years ago to the present. The classification is a catch all term used to refer to any organic microfossils that cannot be assigned to other groups. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.

<span class="mw-page-title-main">Doushantuo Formation</span>

The Doushantuo Formation is a geological formation in western Hubei, eastern Guizhou, southern Shaanxi, central Jiangxi, and other localities in China. It is known for the fossil Lagerstätten in Zigui in Hubei, Xiuning in Anhui, and Weng'an in Guizhou, as one of the oldest beds to contain minutely preserved microfossils, phosphatic fossils that are so characteristic they have given their name to "Doushantuo type preservation". The formation, whose deposits date back to the Early and Middle Ediacaran, is of particular interest because it covers the poorly understood interval of time between the end of the Cryogenian geological period and the more familiar fauna of the Late Ediacaran Avalon explosion, as well as due to its microfossils' potential utility as biostratigraphical markers. Taken as a whole, the Doushantuo Formation ranges from about 635 Ma at its base to about 551 Ma at its top, with the most fossiliferous layer predating by perhaps five Ma the earliest of the 'classical' Ediacaran faunas from Mistaken Point on the Avalon Peninsula of Newfoundland, and recording conditions up to a good forty to fifty million years before the Cambrian explosion at the beginning of the Phanerozoic.

Phosphatization, or phosphatic fossilization, refers to the process of fossilization where organic matter is replaced by abundant calcium-phosphate minerals. It has occurred in unusual circumstances to preserve some extremely high-resolution microfossils in which careful preparation can even reveal preserved cellular structures. Such microscopic fossils are only visible under the scanning electron microscope.

Markuelia is a genus of fossil worm-like bilaterian animals allied to Ecdysozoa and known from strata of Lower Cambrian to Lower Ordovician age containing five species.

<i>Vernanimalcula</i> Fossil of possible very early bilateral animal

Vernanimalcula guizhouena is an acritarch dating from 600 to 580 million years ago; it was between 0.1 and 0.2 mm across. Vernanimalcula means "small spring animal", referring to its appearance in the fossil record at the end of the Marinoan Glaciation and the belief upon discovery it was an animal.

Vendobionts or Vendozoans (Vendobionta) are a proposed very high-level, extinct clade of benthic organisms that made up of the majority of the organisms that were part of the Ediacaran biota. It is a hypothetical group and at the same time, it would be the oldest of the animals that populated the Earth about 580 million years ago, in the Ediacaran period. They became extinct shortly after the so-called Cambrian explosion, with the introduction of fauna formed by more recognizable groups and more related to modern animals. It is very likely that the whole Ediacaran biota is not a monophyletic clade and not every genus placed in its subtaxa is an animal.

Namacalathus is a problematic metazoan fossil occurring in the latest Ediacaran. The first, and only described species, N. hermanastes, was first described in 2000 from the Nama Group of central and southern Namibia.

The Ediacaranbiota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period. These were enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms. The term "Ediacara biota" has received criticism from some scientists due to its alleged inconsistency, arbitrary exclusion of certain fossils, and inability to be precisely defined.

Philip Conrad James Donoghue FRS is a British palaeontologist and Professor of Palaeobiology at the University of Bristol.

The preservational mode of the Doushantuo formation involves very early phosphatisation on a cellular level - with cells being replaced by phosphate before they degrade.

A microbial mat is a multi-layered sheet of microorganisms, mainly bacteria and archaea, or bacteria alone. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few survive in deserts. A few are found as endosymbionts of animals.

The Cambrian explosion, Cambrian radiation,Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately 538.8 million years ago in the Cambrian Period of early Paleozoic when there was a sudden radiation of complex life and practically all major animal phyla started appearing in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.

The urbilaterian is the hypothetical last common ancestor of the bilaterian clade, i.e., all animals having a bilateral symmetry.

<span class="mw-page-title-main">Deuterostome</span> Superphylum of bilateral animals

Deuterostomia are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some examples of deuterostomes include vertebrates, sea stars, and crinoids.

Holozoa is a group of organisms that includes animals and their closest single-celled (protist) relatives, but excludes fungi and all other organisms. It is a monophyletic group or clade, a lineage consisting of all descendants of a common ancestor. Among these descendants, the protists are of high interest because of their close relationship to animals: in the search for the genes responsible for animal multicellularity within these protists, they help elucidate the nature of the unicellular ancestor of animals.

The Lantian Formation is a 150-meter-thick sequence of rocks deposited in Xiuning County, Anhui Province in southern China during a 90-million-year epoch in the Ediacaran period. Its algal macrofossils are the oldest large and complex fossils known.

Olivooides is an extinct, sphere-shaped microfossil from Cambrian strata. Fossils are currently known only from China. Olivooides was approximately 600‐870 μm in diameter. It was an egg with a large yolk content. Fossils from Shaanxi, China can be found in the cleavage, gastrulation, organogenesis, cuticularization, pre‐hatching, post‐hatching and subsequent growth stages of development. This fossil is a result of soft-bodied preservation. Olivooides has pentaradial symmetry and is usually preserved by calcium phosphate endocast. The internal structure is rarely preserved. It has no larval stage, so it likely had a quick and direct development.

Shuhai Xiao is a Chinese-American paleontologist and professor of geobiology at Virginia Tech, Blacksburg, Virginia, U.S.A.

References

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↑ Yao, X.; Han, J.; Jiao, G. (2011). "Early Cambrian epibolic gastrulation: A perspective from the Kuanchuanpu Member, Dengying Formation, Ningqiang, Shaanxi, South China". Gondwana Research. 20 (4): 844–851. Bibcode:2011GondR..20..844Y. doi:10.1016/j.gr.2011.04.003.
↑ Huldtgren, T.; Cunningham, J. A.; Yin, C.; Stampanoni, M.; Marone, F.; Donoghue, P. C. J.; Bengtson, S. (2011). "Fossilized Nuclei and Germination Structures Identify Ediacaran "Animal Embryos" as Encysting Protists". Science. 334 (6063): 1696–1699. Bibcode:2011Sci...334Q1696H. doi:10.1126/science.1209537. PMID 22194575. S2CID 39813961.
↑ Zhang, X. G.; Pratt, B. R. (2014). "Possible Algal Origin and Life Cycle of Ediacaran Doushantuo Microfossils with Dextral Spiral Structure". Journal of Paleontology. 88: 92–98. doi:10.1666/13-014. S2CID 140625640.
↑ Tanke, D.H.; Brett-Surman, M.K. (2001). "Introduction". In D.H. Tanke; K. Carpenter (eds.). Evidence of Hatchling and Nestling-Size Hadrosaurs (Reptilia:Ornithischia) from Dinosaur Provincial Park (Dinosaur Park Formation: Campanian), Alberta, Canada. pp. 206-218. Mesozoic Vertebrate Life—New Research Inspired by the Paleontology of Philip J. Currie. Bloomington: Indiana University Press. pp. xviii + 577 pp.